Oslo CTM3 Chemistry

The chemistry of the Oslo CTM3 covers both tropospheric and stratospheric chemistry, treated by seperate modules (see menu on the left). The kinetics is based on JPL 2006, while the photodissociation coefficients are calculated on-line using the Fast-JX method.

Integrating chemistry
The numerical integration of chemical kinetics is done applying the Quasi Steady State Approximation (QSSA) (Hesstvedt et al., 1978), using three different integration methods depending on the chemical lifetime of the species.

How it is done
The Oslo CTM3 chemistry is divided into two schemes, one for the troposphere and one for the stratosphere. The tropospheric code is stand-alone, but the stratospheric code needs the tropospheric chemistry module to work. The historical reason for two schemes is that limitations in computational power previously meant one could only afford to study either the troposphere or the stratosphere.
Nevertheless, such a division is possible since species with negligible chemical conversions in the stratosphere (troposphere) do not have to be treated in the stratospheric (tropospheric) chemistry scheme. Only compounds important for both tropospheric and stratospheric chemistry are treated in both schemes.

To select which chemical scheme to apply, we use the tropopause height, based on the potential vorticity and potential temperature of the meteorological data.